sound out - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Seek the views or intentions of, as in We'd better sound out Mom about who's using the station wagon, or Let's sound out the staff before we decide which week we should close for vacation. This expression derives from sound meaning “to measure the depth of water by lowering a line or lead.” It was transferred to other kinds of inquiry in the late 1500s, but out was not added for several centuries.